The present invention relates to a method and apparatus for heating thick-walled glass tubes, particularly quartz tubes intended for the manufacture of optical fibres.
One known method of manufacturing optical fibres is based on the use of a glass tube. A layer of pure quartz, SiO.sub.2, is deposited on the inner wall of the tube. Germanium dioxide GeO.sub.2 is also deposited in given layers as a doping agent.
This is effected by introducing silicon tetrachloride SiCl.sub.4, and free oxygen O.sub.2, into the tube, together with germanium tetrachloride, GiCl.sub.4. Silicon dioxide and germanium dioxide and herewith deposited on the inner walls of the tube. In order for these reactions to take place, it is necessary to heat the interior of the tube to a temperature of about 1400.degree. C. This is normally achieved by passing a burner along the length of the tube, while rotating the tube abouts its axis. Firstly, silicon tetrachloride, SiCl.sub.4, and oxygen, O.sub.2, are introduced, whereupon silicon dioxide is deposited and forms a porous layer. When the burner passes over the porous layer, the layer sinters to form a transparent silicon dioxide. This deposition process, is repeated, whereafter germanium tetrachloride is mixed with the silicon tetrachloride gas, so as to develop a correct so-called index profile. Normally, from 30 to 100 layers are applied in the deposition phase. Upon completion of the deposition phase and sintering phase, the temperature of the burner is raised, so as to heat the tube to about 2200.degree. C. At this temperature the surface tensions created cause the tube to draw together, i.e. to collapse. Subsequent to traversing the tube a few times with the burner, the tube forms a rod, i.e. a so-called preform, from which an optical fibre can be drawn.
One known method of increasing the speed of the process during the deposition phase is to transmit high frequency energy with the aid of a coil wound around the tube, for example energy having a frequency of 3-4 MHz, there being formed within a tube a plasma having a temperature of about 10,000.degree. C.
It is necessary to follow the coil with a burner, in order to sinter the deposited material.
Another known method is one in which the pressure within the tube is lowered to about 10 mbar and a microwave energy field is applied to generate arcing in the gas present in the tube, therewith to form a low-temperature plasma without heating the tube directly with microwave energy.
In order to maximise production, it is desired to shorten the time taken to effect each part process, i.e. deposition, sintering, and tube collapse, and to increase the size of the preform so as to reduce the influence of the apparatus set-up times. An obvious way of producing larger preforms is to begin with larger and bigger tubes. However, when larger tubes are used it is difficult to achieve a stable collapse and a proportional increase in the collapse time, because quartz glass is a remarkably poor conductor. In order to achieve a collapse, it is necessary to heat the tube uniformly to a temperature of about 2200.degree. C.
In all known processes the tube is heated to the requisite temperature with the aid of a gas flame.
All methods are therefore contingent on the poor thermal conductivity of the glass.